Multi-color imaging systems, such as color printers and color copiers, are widely available, and offer various combinations of performance and affordability for almost any application. Technological advancements in ink formulation, print medium development, and printer drivers allow the production of sophisticated color documents.
The generation of a printed color document typically begins with a description of the desired image in color space. This description may be generated by a scanner, or it may be generated on a computer using color image creation software. The colors present in an image defined by a workstation or a personal computer typically exist in a device-independent color space. A commonly utilized color space is CIELAB space.
CIELAB space, or more properly, 1976 CIE L*a*b* space, is a tristimulus color space with the coordinates L*, a*, and b*. The central vertical axis (L*) represents lightness, with values from 0 (black) to 100 (white). The two color axes each run from positive to negative. On the a–a′ axis (a*), positive values indicate amounts of red while negative values indicate amounts of green. On the b–b′ axis (b*), yellow is positive, and blue is negative. For both the a–a′ axis and the b–b′ axis, zero is neutral gray. A single specific color can be uniquely identified with a value for each color axis, and a value for the lightness or grayscale axis. CIE L*a*b* space is device-independent.
In order for an imaging system to print the desired image, the device-independent coordinates must first be mapped to a device-dependent space that is particular to the imaging system used. For many imaging systems, the device-dependent color space is CMYK space (for Cyan, Magenta, Yellow, and black colorants). A given imaging system uses a color map to transform a selected point in device-independent color space to a particular point in CMYK space. This transformed color corresponds to the amount of cyan, magenta, yellow, and black colorant required in device-dependent color space to generate the color defined in device-independent color space.
The color map conversion of device-independent color to device-dependent color typically takes into account certain physical constraints of the imaging system (for example ink drop size, ink density, ink behavior on the printing substrate), and typically applies a linearization function to each color channel. There may also be mathematical procedures built into the color map to maximize imaging efficiency and to enhance image quality, such as dithering and half-pixel interpolation. These aspects of color mapping and multi-color imaging have been described in detail previously, and will not be discussed further here.
Unfortunately, a variety of factors can impact the quality of the final image. By final image is meant the appearance of the image, including color, that is stable with respect to time. Variations in colorant density, colorant composition, colorant delivery rate, and media composition can result in wide differences in the appearance of an image after printing. For example, several post-printing modifications of the printed output, such as applying a protective sealant or laminating the output, typically also affect the color of the final image. A printed image can also “settle” , or change color, after printing. This color shift is in part due to the mobility of dye molecules on and within the print medium as the freshly applied colorant ages. Inkjet dyes, in particular, are typically applied as aqueous solutions. Until such ink is completely dry, the migration of dye molecules on or in the print medium can continue to alter the appearance of the image. Differential migration and diffusion of individual color components in a multi-color image can also lead to a blurring of details, darkening of the image, or color shift in the image as a whole.
As the nature and extent of dye mobility is to a large extent controlled by the rate of drying of the ink, it should be appreciated that ambient humidity levels have a strong effect on color shift. Likewise, ambient temperature can have a significant effect on the degree and magnitude of color shift observed. In general, higher humidity levels and higher temperatures result in more rapid and more substantial deviations from original color levels.
Table 1 provides an example of the type and magnitude of color shift that can be observed in a printed image using standard inks. Eight series, each having ten test patches, were printed in the colors blue, compK (composite black made from cyan, magenta, and yellow), cyan, green, magenta, red, skin tone, and yellow. The test patches were printed using an inkjet printer and optically scanned to determine their initial color characteristics. The test patches were then placed in a test chamber kept at 35° C. and 80% relative humidity for 4 days. After incubation, the test patches were again optically scanned.
Changes in test patch appearance were measured in units of ΔE (Delta E) according to the following formula:ΔE=√{square root over ((ΔL*)2+(Δa*)2+(Δb*)2)}{square root over ((ΔL*)2+(Δa*)2+(Δb*)2)}{square root over ((ΔL*)2+(Δa*)2+(Δb*)2)}where ΔL*=L*1-L*2, Δa*=a*1-a*2, and Δb*=b*1-b*2 In general, a ΔE value of 1 is a just perceptible difference and a ΔE value greater than 1 is generally readily distinguished. For comparison purposes, a change of 100 Delta E units represents the difference between a perfectly black patch and a perfectly white patch.
As shown in Table 1, many of the test patches exhibited a color shift of as much as 10 Delta E units, some as high as 16 Delta E units. This represents a substantial change in the appearance of the printed output after settling under the stated environmental conditions (35° C. and 80% relative humidity for 4 days).
TABLE 1Color shifts after four days at 35° C. and 85% relative humidityColor Shift (ΔE)Patch No.12345678910Blue7.211.213.214.717.917.014.212.311.617.1CompK4.67.38.88.28.36.76.66.97.711.0Cyan1.71.61.41.63.13.53.63.73.74.7Green6.89.210.612.212.210.79.38.07.210.5Magenta10.613.113.415.015.916.114.512.511.04.7Red7.710.813.615.614.512.19.16.85.811.0Skin9.38.88.56.6——————Yellow7.07.57.27.57.87.97.57.26.33.5
Current imaging system calibration methods compensate for settling in only a limited fashion, and only under nominal temperature and humidity levels. Deviations from these ideal conditions typically result in imperfect correction, and subsequent color drift in the final image. There is also no rapid and convenient way to allow for changes to the appearance of a printed image due to variations in print medium, lamination, or other image-altering process. What is needed is a method of correcting color output in an imaging system that actively compensates for a variety of post-printing changes to the image color.